1. Field of the Invention
The present invention relates to a shift control system for automatic transmission, especially to a hydraulic control system for up-shift.
2. Description of the Prior Art
In an automatic transmission for a vehicle, there is provided a gear transmission mechanism which has a plurality of engaging devices acting under oil pressure, and a prescribed gear position is selected based on the running condition of the vehicle, so as to change the gear ratio by engaging and disengaging the engaging devices. As shown in FIG. 12, there is set a gear shift line L showing the relationship between the opening of the throttle valve (throttle opening TVO) based on the operation of the accelerator pedal and the vehicle speed Ns, and the gear position is changed when the running condition of the vehicle changes across the gear shift line L.
For example, when the vehicle speed increases while the throttle opening TVO is substantially constant as shown by arrow xe2x80x9caxe2x80x9d or is changing gently, oil pressure for the engaging devices are controlled based on an up-shift command. An up-shift while the throttle opening is substantially constant or slowly changing is referred to as an auto up-shift, with cases of an engine speed of around 3000 rpm being particularly prevalent.
This oil pressure control during gear shifting is performed as a combination of open control where oil pressure is increased according to increases in the input torque of the gear transmission mechanism and feedback control corresponding to gear shift transitions. The object of feedback control is to prevent the occurrences of shocks due to large changes in output shaft torque of a gear transmission mechanism during gear shifting, and sets a target gear ratio for the process of switching from a gear ratio of the gear position before shifting to a gear ratio of the gear position after shifting, and ensures that this target gear ratio is maintained.
FIG. 13 shows the relationship between gear ratio, output shaft torque of the gear transmission mechanism, and oil pressure command value at the time of auto up-shift.
When there is a shift command to shift from a first gear position to a second gear position, an open control component POP having a gradient based on input torque at this time is first set as an oil pressure command value in an inertia phase after a standby phase. Changes in gear ratio due to the open control component is as shown by the broken line in FIG. 13A. However, in order to suppress the amount of change (step difference) in the output shaft torque shown in FIG. 13B to a predetermined amount B so as to absorb energy of the change in torque, the target gear ratio is set to that shown by the solid line in FIG. 13A.
As shown in FIG. 13C, the oil pressure command value is set at Pc, which is obtained by correction of the open control component POP using the feedback control component PFB, so as to make the gear ratio in line with the aforementioned target gear ratio.
Accordingly, intermediate large changes in output shaft torque are suppressed so as to give gear shift with no shock.
However, in the related art, control of oil pressure combining this open control and feedback control has been carried out uniformly during gear shifting. This invites the problem of shocks under different gear shift conditions, although shocks are effectively prevented during auto up-shift.
Namely, an up-shift from the first gear position to the second gear position also occurs when the throttle opening TVO changes across the gear shift line L, as shown by arrow b in FIG. 12, when a foot depressing the accelerator pedal is released.
Such up-shift due to foot release starts at a high engine speed in the region of 6000 rpm, as compared to auto up-shift.
In this case, the input torque is reduced due to the reducing of throttle opening TVO, the gradient of the open control component POP becomes small, the rate of the rise in oil pressure becomes lower, and the engine speed is in the high region. Therefore, the gear ratio changes as shown by the broken line in FIG. 14A, and the gear shifting tends to become delayed.
An oil pressure command value Pc, which is obtained by correction of an open control component POP using the feedback control component PFB for making the gear ratio in line with the target gear ratio, is therefore large as shown by the solid line in FIG. 14C compared with a case in which only an open control is adopted.
During this time, as shown in FIG. 14B, output shaft torque of the gear transmission mechanism rises smoothly at the beginning because the gradient of the open control in the initial period of the gear shift in the inertia phase starting at time t0 is small, and then abruptly rises from the latter half to the final period. If it is assumed that the input torque is the same as during auto up-shift, the energy it is intended to absorb for the change in torque is also the same. Therefore, the energy that is not absorbed in the initial gear shift period has to be absorbed in the latter half.
The step difference of the output shaft torque occurring at a gear shift end time t1 therefore becomes particularly large and a shock occurs.
It is therefore the object of the present invention to provide a shift control system for automatic transmission in which shocks do not occur even at the up-shift caused by releasing a foot from accelerator pedal.
In order to achieve this object, the present invention provides a shift control system for automatic transmission, the transmission being equipped with engaging devices in a gear transmission mechanism, and when up-shifting from a first gear position to a higher second gear position, oil pressure command value for an engaging device used in the up-shift is made up of an open control component, the gradient of which is determined based on input torque, and a feedback control component based on a target gear ratio, wherein the feedback control component is decreased when the amount of change in a throttle opening at the time of the up-shift is equal to or greater than a predetermined threshold value.
The phenomena where a shock occurs due to excessive feedback corresponding to the gradient of the open control component is prevented at the time of up-shift caused by releasing a foot from the accelerator pedal, when throttle opening is dramatically reduced so that the input torque is also reduced so as to make the gradient of the open control component small.